1. Field of the Invention
The present invention relates to a gain-coupled distributed feedback semiconductor laser and, more particularly, to a gain-coupled distributed feedback semiconductor laser used as a light source for optical fiber communication or the like.
2. Background Art
In recent years, a gain-coupled distributed feedback semiconductor lasers (DFB-LDs) have been used as a light source for transmission of a large amount of data in optical fiber communication or the like. For example, a gain-coupled distributed feedback semiconductor laser is disclosed in IEEE JOURNAL OF QUANTUM ELECTRONICS. VOL. 27, NO. 6 JUNE 1991, pp. 1732-1735, Nakano et al., “Reduction of Excess Intensity Noise Induced by External Reflection in a Gain-Coupled Distributed Feedback Semiconductor Laser”.
A gain-coupled DFB-LD is formed, for example, by using a p-InP substrate. An example of the structure of the DFB-LD as viewed in a section in a direction from a resonator facet side will be described. A p-InP cladding layer is provided on the p-InP substrate (“p-” denotes the p-type and “n-” denotes the n-type in the following description). An InGaAsP-MQW active region, an n-InP intermediate layer, an n-InGaAs absorption-type diffraction grating and an n-InP capping layer laid one on another with a predetermined thickness are formed on the p-InP cladding layer. A current blocking layer including a p-InP blocking layer, an n-InP blocking layer and a p-InP blocking layer laid one on another are embedded on both sides of the above-described layers. An n-InP contact layer is provided on the current blocking layer and the n-InP capping layer. A silicon oxide film is formed on the n-InP contact layer. An opening is provided in the silicon oxide film so that the upper surface of the n-InP contact layer is exposed. A Ti/Au top electrode (n-electrode) is provided on the silicon oxide film so as to fill the opening. A Ti/Au bottom electrode (p-electrode) is provided on the back surface of the p-InP substrate.
A front facet (laser light emission facet ) is provided at one end of the resonator of the above-described DFB-LD, and a rear facet is provided at the other end. Low-reflectivity coating films are formed on the front and rear facets.
The above-described n-InGaAs absorption-type diffraction grating selects laser light of a predetermined oscillation wavelength generated in the MQW active region to improve single-mode yield at the oscillation wavelength.
In the above-described conventional gain-coupled DFB-LD, there is a need to provide a low-reflectivity coating on each of the front and rear facets in order to achieve the single-mode oscillation operation. Laser light is evenly emitted from the front and rear facets with the low-reflectivity coatings. Therefore the efficiency of optical output from the front facet to injected current is low.
If a low-reflectivity coating and a high-reflectivity coating are provided on the front facet and the rear facet, respectively, (that is, asymmetric coatings are provided) to obtain a sufficient optical output from the front facet, laser light is oscillated in two modes. In such a case, a stable operation with a single wavelength cannot be achieved and the laser cannot be used for optical fiber communication.